[ARM] Clear the constant pool cache on explicit .ltorg directives

Multiple ldr pseudoinstructions with the same constant value will
reuse the same constant pool entry. However, if the constant pool is
explicitly flushed with a .ltorg directive, we should not try to
reference constants in the previous pool any longer, since they may
be out of range.

This fixes assembling hand-written assembler source which repeatedly
loads the same constant value, across a binary size larger than the
pc-relative fixup range for ldr instructions (4096 bytes). Such
assembler source already uses explicit .ltorg instructions to emit
constant pools with regular intervals. However if we try to reuse
constants emitted in earlier pools, they end up out of range.

This makes the output of the testcase match what binutils gas does
(prior to this patch, it would fail to assemble).

Differential Revision: https://reviews.llvm.org/D32847